The present invention relates to an improvement in the preparation of an olefin compound, more particularly, to an improvement in the step of, in the process for the synthetic preparation of an olefin compound having usefulness as agricultural chemicals, perfumes, biologically active substances and the like by the so-called Wittig reaction for the synthesis of an olefin compound, isolating the olefin compound as the product from the reaction mixture with high efficiency.
As is known, the Wittig reaction is a very efficient route for the synthetic preparation of an olefin compound, in which the carbonyl group in a carbonyl compound is converted specifically and position-selectively into a vinyl group. In a typical procedure utilizing the Wittig reaction, as is described, for example, in Org. React., volume 14, page 270 (1965), an organic phosphonium compound is subjected to a treatment with a basic compound to form a phosphorane compound which is admixed and reacted with a carbonyl compound to give a mixture of an olefin compound and a phosphine oxide compound.
The above mentioned process of the Wittig reaction, however, has a serious problem that, since the reaction mixture after the reaction contains the phosphine oxide compound as a by-product in an equimolar amount to the desired olefin compound, isolation of the desired olefin compound from the reaction mixture is very troublesome. In principle, the olefin compound can be separated from the phosphine oxide compound by filtration. Difficulties are encountered, however, in practicing the filtration process in a large industrial scale because special and expensive facilities are required therefor and pollution of the working environment is more or less unavoidable thereby adverse influences on the workers' health.
A method for facilitating separation of the phosphine oxide compound from the reaction mixture is known, in which the phosphine oxide compound is soluble in an aqueous alkali or acid solution when the phosphine compound of the formula Ph.sub.2 PAr, in which Ph is a phenyl group and Ar is a group expressed by the formula C.sub.6 H.sub.4 COOH or --C.sub.6 H.sub.4 N(CH.sub.3).sub.2, respectively, is used as the starting material in the reaction described in Chem. Ber., volume 99, page 2663 (1966) and J. Chem. Soc., page 2130 (1961) or the so-called Wittig-Horner-Emmons reaction described in Chem. Rev., volume 74, page 87 (1974). This method is advantageous in some respects but several disadvantages are involved therein in that the organic phosphonium salt is a special compound which can be synthesized only by a lengthy synthetic procedure and there may be an adverse influence on the environment due to the alkaline or acidic waste water containing the water-soluble phosphine oxide compound produced in large volumes when the process is practiced in an industrial scale.
On the other hand, it is of course a possible way that the reaction mixture before removal of the phosphine oxide compound is subjected to a procedure of distillation so as to distil out the olefin compound as the desired product by utilizing the difference in the boiling point between the olefin compound and the phosphine oxide compound. A problem in this method is that the boiling point of the reaction mixture under distillation is subject to elevation of boiling point as the distillation proceeds so as to cause troubles due to the stereochemical changes in the molecular structure of the olefin compound, for example, by isomerization or polymerization of the olefin compound caused by the prolonged heating of the reaction mixture as a consequence of the decreased velocity of distillation of the olefin compound. Accordingly, it is eagerly desired to develop a novel and efficient method for the isolation of an olefin compound from the reaction mixture after the Wittig reaction to synthesize the olefin compound.